The performance of an internal combustion engine can be improved by the use of dual camshafts, one to operate the intake valves of the various cylinders of the engine and the other to operate the exhaust valves. Typically, one of such camshafts is driven by the crankshaft of the engine, through a sprocket and chain drive or a belt drive, and the other of such camshafts is driven by the first, through a second sprocket and chain drive or a second belt drive. Alternatively, both of the camshafts can be driven by a single crankshaft powered chain drive or belt drive. Engine performance in an engine with dual camshafts can be further improved, in terms of idle quality, fuel economy, reduced emissions or increased torque, by changing the positional relationship of one of the camshafts, usually the camshaft which operates the intake valves of the engine, relative to the other camshaft and relative to the crankshaft, to thereby vary the timing of the engine in terms of the operation of intake valves relative to its exhaust valves or in terms of the operation of its valves relative to the position of the crankshaft.
U.S. Pat. No. 5,002,023 describes a VCT phaser having a pair of oppositely acting hydraulic cylinders which operate with a hydraulic system which includes appropriate hydraulic flow elements to selectively transfer hydraulic fluid from one of the cylinders to the other to advance or retard the circumferential position of a camshaft relative to a crankshaft. The VCT system utilizes a control valve in which the exhaustion of hydraulic fluid from one or another of the oppositely acting cylinders is permitted by moving a spool within the valve one way or another from its centered or null position. The movement of the spool occurs in response to an increase or decrease in control hydraulic pressure on one end of the spool and the relationship between the hydraulic force on the one end and an opposing direct mechanical force from a compression spring.
However, when the engine is shut down, oil can leak from the VCT phaser. Consequently, during an engine start, before the engine oil pump generates sufficient oil pressure, the lack of controlling oil pressure in the chambers can allow the phaser to oscillate excessively, producing noise and possibly damaging the mechanism. Hence, it is desirable to have the phaser locked in a particular position while the engine is being started. One known solution is to employ a lock pin that can lock the phaser in a specific phase angle position relative to the crankshaft when insufficient oil exists in the chambers. These lock pins are typically spring loaded to engage and are released using engine oil pressure.
US Patent Application 2004/0055550A1 discloses a VCT system in which the spool valve used to control the VCT mechanism is also used to actively control the locking pin. In other words, a VCT system that utilizes a spool valve for controlling the VCT mechanism is actively used to control a locking pin. The spool valve utilises a spool having multiple lands such that the position of the spool directly influences whether source oil is supplied to both the locking pin and to either the advance or retard chambers of the phaser. The VCT mechanism provides for adjusting and maintaining an angular relationship between a camshaft and a crankshaft or another shaft using a pressurized fluid, and has a phaser using the pressurized fluid for adjusting and maintaining the angular relationship, the pressurized fluid flows from a fluid source to a fluid sink. The VCT mechanism has: a locking pin disposed to engage a recess with the pressurized fluid acting on the lock pin to thereby disengage the lock pin from the recess; a spool valve having three lands which co-operate with fluid passageways to control the flow of the pressurized fluid for adjusting and maintaining the angular relationship, and which also control the timing of the pressurized fluid flowing from the fluid source toward the lock pin and from the lock pin toward the fluid sink.
The spool position is monitored and controlled by solenoid closed loop control. Under normal operating conditions a hydraulically actuated or supported VCT with an active lock pin will not unlock until sufficient oil pressure is available and there is a signal from the ECU (engine control unit). Provided both are present, the spool will open the oil passageway to the lock pin allowing pressurized oil to unlock the pin. A disadvantage of this arrangement is that every time the spool returns to a base position (to return the VCT to base position), the lock pin oil pressure is shut-off and the pin engages. During the life of the engine, the lock pin will engage and disengage numerous times inducing a high wear rate on the lock pin, its bore and the pin seat. It also induces a delayed pin unlock which in turn delays the phaser response time.